It is known to those skilled in the art that ethers, including unsymmetrical ethers, may be prepared by reacting an alcohol with another alcohol to form the desired product. The reaction mixture, containing catalyst and/or condensing agent may be separated and further treated to permit attainment of the desired product. Such further treatment commonly includes one or more distillation operations.
Methyl tert-butyl ether is finding increasing use as a blending component in high octane gasoline as the current gasoline additives based on lead and manganese are phased out. Currently all commercial processes for the manufacture of methyl tert-butyl ether are based upon the liquid-phase reaction of isobutylene and methanol (Eq. 1), catalyzed by a cationic ion-exchange resin with an organic polymer backbone (see, for example: Hydrocarbon Processing, Oct. 1984, p. 63; Oil and Gas J., Jan. 1, 1979, p. 76; Chem. Economics Handbook-SRI, Sept. 1986, p. 543-7051P). The cationic ion-exchange resins used in MTBE synthesis normally have the sulfonic acid functionality and a cross-linked styrene-divinylbenzene polymer backbone (see: J. Tejero, J. Mol. Catal., 42 (1987) 257; C. Subramamam et al., Can. J. Chem. Eng., 65(1987) 613). ##STR2##
With the expanding use of MTBE as an acceptable gasoline additive, a growing problem is the availability of raw materials. Historically, the critical raw material is isobutylene (Oil and Gas J., Jun. 8, 1987, p. 55). It would be advantageous, therefore, to have a process to make MTBE that does not require isobutylene as a building block. It would be advantageous to have an efficient process for making MTBE by reaction of methanol with tertiary butyl alcohol, since t-butanol (tBA) is readily available commercially through isobutane oxidation.
In U.S. Pat. No. 4,822,921, to Texaco Chemical Co., there is described a method for preparing alkyl tertiary alkyl ethers which comprises reacting a C.sub.1 -C.sub.6 primary alcohol with a C.sub.4 -C.sub.10 tertiary alcohol over a catalyst comprising an inert support impregnated with phosphoric acid.
U.S. Pat. No. 4,827,048, to Texaco Chemical Co., describes a method for preparing alkyl tertiary alkyl ethers from the same reactants using a heteropoly acid on an inert support.
U.S. Pat. No. 5,099,072, to Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers over an acidic montmorillonite clay catalyst which possesses very specific physical parameters.
U.S. Pat. No. 5,081,318, to Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers by reacting a C.sub.1 -C.sub.6 primary alcohol with a C.sub.4 -C.sub.10 tertiary alcohol over a catalyst comprising a fluorosulfonic acid-modified zeolite.
U.S. Pat. No. 5,059,725, to Texaco Chemical Co., discloses a method for preparing alkyl tertiary alkyl ethers, including ethyl tertiary butyl ether, from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols over a catalyst comprising ammonium sulfate or sulfuric acid on a Group IV oxide.
U.S. Pat. No. 5,157,162, to Texaco Chemical Co., discloses a fluorosulfonic acid-modified clay catalyst for the production of aliphatic ethers from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols.
In U.S. Pat. No. 5,162,592, to Texaco Chemical Co. there is described a method for producing alkyl tertiary alkyl ethers from C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols using a multimetal-modified catalyst.
A hydrogen fluoride-modified montmorillonite clay catalyst is employed in U.S. Pat. No. 5,157,161, to Texaco Chemical Co., to produce alkyl tertiary alkyl ethers.
In U.S. Pat. No. 5,183,947, to Texaco Chemical Co., fluorophosphoric acid-modified clays are employed as catalysts in a method to produce alkyl tertiary alkyl ethers.
In allowed U.S. Ser. No. 07/917,218, assigned to Texaco Chemical Co., there is disclosed the use of a super acid alumina or a faujasite-type zeolite to produce alkyl tertiary alkyl ethers.
Allowed U.S. Ser. No. 07/878,121, to Texaco Chemical Co., discloses the use of a haloacid-modified montmorillonite clay catalyst to convert C.sub.1 -C.sub.6 primary alcohols and C.sub.4 -C.sub.10 tertiary alcohols to alkyl tertiary alkyl ethers.
Fluorophosphoric acid-modified zeolites are employed in allowed U.S. Ser. No. 07/917,885, to Texaco Chemical Co., to produce alkyl tertiary alkyl ethers.
Sulfur promoted and "superacid" catalysts are known in the art. Superacid catalysts are particularly desirable for reactions where lower temperatures are favored. In an article titled "Design of Sulfur-Promoted Solid Superacid Catalyst" by K. Tanabe and T. Yamaguchi in "Successful Design of Catalysts," Inui, T. (Editor) Elsevier Science Publishers B. V., Amsterdam, 1988, p. 99, there is a discussion of the extremely high catalytic activities of sulfur-promoted superacids, including the factors controlling super acidity. Solid superacids such as SO.sub.4.sup.2- /ZrO.sub.2, SO.sub.4.sup.2- /TiO.sub.2 and SO.sub.4.sup.2-2 /Fe.sub.2 O.sub.3 have been reported to exhibit extremely high catalytic activities for acylation and alkylation of aromatics, esterification of phthalic acid, skeletal isomerization of paraffins, dehydration of alcohols, polymerization of alkyl vinyl ethers, liquefaction of coal and rearrangement of oximes.
It is noted that the strength of the superacid depends on the extent of losing the S.dbd.O double bond character by an electronic shift from an adsorbed basic molecule to the sulfur complex. The larger the shift, the higher the acid strength.
The acid strength can vary depending on the preparation method, however, the acid strength of SO.sub.4.sup.2- /ZRO.sub.2 is apparently 10,000 times higher than that of 100% H.sub.2 SO.sub.4. The effect of the addition of SO.sub.4.sup.2- on catalytic activity is surprisingly large.
Ibid., page 101, there is a comparison of the acidities achieved by introduction of various sulfur compounds, such as ammonium sulfate, SO.sub.3, SO.sub.2, or H.sub.2 S, onto ZrO.sub.2, TiO.sub.2, Fe.sub.2 O.sub.3, Al.sub.2 O.sub.3, SnO.sub.2, SiO.sub.2 and Bi.sub.2 O.sub.3. From a comparison of experimentally obtained spectra of sulfur-promoted oxides under various conditions, it was observed that whatever the starting sulfur compounds are, once they were oxidized on the surface of ZrO.sub.2, TiO.sub.2 and Fe.sub.2 O.sub.3, they form a structure in which the presence of two covalent SO double bonds is characteristic. The structure is responsible for the generation of the strong acidity and a central metal cation plays as a Lewis acid site. The formation is basically a chemical reaction between SO.sub.4.sup.2-, SO.sub.2 or SO.sub.3 and the oxide surfaces to form the definite structure in which two covalent bonds are involved.
Results indicated that when a basic molecule is adsorbed on the central metal cation, it tends to reduce the bond order of SO from a highly covalent double-bond character to a lesser double-bond character.
The stability of the catalyst upon hydrogen reduction at various temperatures, and the facility of regeneration upon reoxidation was tested using the dehydration of 2-propanol as the test reaction. The catalytic activity decreased with increase in reduction temperature from 100.degree. to 450.degree. C. It was theorized that the activity loss by reduction at lower temperatures might be the result of the removal of surface oxygens since recovery of the catalysts by oxidation was possible to varying extents.
It was observed that only ZrO.sub.2, TiO.sub.2 and Fe.sub.2 O.sub.3 gave strong acidity by sulfur promotion, possibly because the number of acid sites thus obtained may be limited by the surface area of the oxides.
In an article by O. Saur et al., J. Catal., 99, (1986) 104-110 titled "The Structure and Stability of Sulfated Alumina and Titania," sulfated alumina and titania were studied using infrared spectroscopy and a vacuum microbalance with the aim of determining the structure of the surface sulfate, its thermal stability, and its reducibility in H.sub.2. It was concluded that the sulfated TiO.sub.2 or Al.sub.2 O.sub.3 has a structure resembling (M.sub.3 O.sub.3)S.dbd.O [M.dbd.Al or Ti], whereas in the presence of H.sub.2 O or excess surface OH groups, this is converted to ##STR3## type groups, thus accounting for the increased Bronsted activity. Finally, the sulfated Al.sub.2 O.sub.3 surface was found to be both more thermally stable and more resistant to reduction in H.sub.2 than the sulfated TiO.sub.2 and the authors state, "sulfates of titania are known to be relatively unstable."
In Catalysis Today, 5 (1989) 493-502 there is an article titled "n-Butane Isomerization on Solid Superacids," by J. C. Yori et al., in which the use of ZrO.sub.2 /SO.sub.4.sup.2- to isomerize n-butane and method of preparation of ZrO.sub.2 /SO.sub.4.sup.2- is discussed. The ZrO.sub.2 /SO.sub.4.sup.2- was calcinated at between 773.degree. K. and 933.degree. K. and optimum catalytic activity was found where calcination took place around 893.degree. K.
Ishida et al. report in Chem. Lett., 1869, 1988 on the "Acid Property of Sulfur-Promoted Zirconium Oxide on Silica as Solid Superacid." Here it was concluded that the higher acid strength of the catalyst can best be achieved after the crystal growth of the supported oxide, and that a tetragonal form of ZrO.sub.2 grows extensively when the amount of ZrO.sub.2 loaded becomes large. This relationship between crystal growth and generation of acidity may be of significance in designing a catalyst having a higher number of acid sites.
More recently, in an article titled "Recent Progress in Solid Superacid," in Applied Catalysis, 61 (1990) 1-25, T. Yamaguchi reviews literature on solid superacids including a discussion of mounted acids, combined acids, and sulfate-promoted metal oxides. It is noted at page 13 that sulfate-promoted metal oxides are useful as catalysts for skeletal isomerization of paraffins, polymerization of ethers, acetylation, benzolation and esterification.
Sulfate-containing solid superacids have been employed for acid-catalyzed hydrocarbon conversions by I. Rodriguez-Ramos et al., Division of Petroleum Chemistry, American Chemical Society, New York City Meeting, Aug. 25-30, 1990, preprints, p. 804. Here, a number of alumina and carbon carriers that had been impregnated with aqueous solutions of iron or zirconium nitrates, then reimpregnated with ammonium sulfate solutions, were tested in MTBE synthesis from methanol plus isobutene (Eq. 1). All catalysts were considerably less active than AMBERLYST.RTM. A-15, a styrene-divinylbenzene polymer with sulfonic acid functionality. This difference in activity appears from the data to be a factor of about 40, even where the AMBERLYST.RTM. A-15 was evaluated at a lower esterification temperature (70.degree. C. versus 95.degree.-120.degree. C.).
There is a discussion titled "Dehydration of Alcohols Catalyzed by Metallic Sulphates Supported on Silica Gel," in J. Chem. Soc. Perkin Trans. I, 1989, 707, authored by T. Nishiguchi and C. Kamio. In this work metallic sulphates and hydrogen sulphates supported on silica gel efficiently catalyzed dehydration of secondary and tertiary alcohols under mild conditions. The dehydration catalytic activity of the sulphates and hydrogen sulphates was examined in the case of cyclododecanol. The sulphates of Ce, Ti and Fe were most active. Silica gel was essential for the efficient dehydration in each case.
The indication was that this type catalyst was unsuitable for primary alcohols. On page 709, Col. 1, lines 3-5, it is stated that primary alcohols failed to react.
The authors suggest that the greater the Lewis acidity of a sulphate, the greater its activity on silica gel and, further, that the proton liberated from hydrogen sulphates presumably contributes to the high activity of the salts because the salts of Na, K and NH.sub.4 on silica gel were inactive.
Silica gel is quite distinct from other forms of silica. The following chart describes the properties of silica gels and indicates how they are different from other forms of silica:
TABLE 1 ______________________________________ Properties of Different Forms of Amorphous Silica Silica Dry Precipi- Silica silica tated from Pyrogenic Property sols gels solution Silica ______________________________________ SiO.sub.2, % 10-15 96.5-99.6 80-90 99.7-99.9 CaO, % na na 0.1-4 na Na.sub.2 O, % 0.1-0.8 0-1 0-1.5 na wt loss, % at 105.degree. C. 50-80 na 5-7 0.5-2.5 at 1200.degree. C. 50-90 2-17.5 10-14 0.5-2.5 ultimate particle 5-100 1-100 10-25 1-100 size, nm aggregate particle 3-25 1-10 2-3 size, .mu.m surface area, m.sup.2 /g 50-700 200-700 45-700 15-400 pH, aqueous 3-5, 8-11 2.3-7.4 4-9 3.5-8 suspension apparent or bulk 1.2-1.4 0.1-0.8 0.03-0.3 0.03-0.12 density, g/cm.sup.3 true density, g/cm.sup.3 2.2-2.3 2.22 2.0-2.1 2.16 refractive index, n.sub.D 1.35- 1.45 1.35-1.45 1.45 1.45 oil absorption, g/g 0.9-3.15 1-3 0.5-2.8 ______________________________________
Silica gels are classified into three types, i.e., regular, intermediate and low-density. Regular density gel is made by gelling in an acid medium, which gives very small particles with high surface area. It generally contains about ca 6 wt % water as surface hydroxyl groups, which imparts a high capacity for water absorption and absorption of other polar molecules. It exhibits a high selectivity for polar molecules and contains a large percentage of small pores, see Kirk Othmer Encyclopedia of Chemical Technology, 3rd Ed., Vol. 20, p. 763.
Intermediate density silica gel has a lower surface area, but larger pore volume. It has a high capacity for water absorption at high humidities.
Low density silica gel has a lower surface area and large average pore diameter. It is usually prepared as a very fine powder of extremely low density.
It is known in the art to use silica gel as a desiccant, as an adsorbent, as a catalyst base, to increase viscosity and thixotropy, for surfactant and optical effects, as a source of reactive silica, for cloud seeding, in chromatographic column packing, as an anticaking agent, and in paper coating.
There is a desire in the art to identify catalysts which would be suitable for producing MTBE. It would be especially desirable to identify catalysts which allow the reaction to be accomplished in one step under relatively mild conditions and show good activity. Although some of the work discussed above suggests the isomerization of alkanes or dehydration of alcohols, there seems to be nothing in the art which would suggest reacting a primary and tertiary alcohol such as methanol and t-butanol over a heterogeneous catalyst where alkylsulfonic acid is bonded to an oxide to produce MTBE and isobutylene. It has now been discovered that a catalyst composition comprising alkylsulfonic acid on a Group III or IV metal oxide provides an active catalyst for MTBE synthesis.